The present invention relates to a polymer electrolyte fuel cell of the room temperature operation type, which is used for a portable power supply, a power supply of electric cars, a domestic cogeneration system, or the like.
A polymer electrolyte fuel cell produces both electric power and heat at the same time by making a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas such as air react electrochemically. The fuel cell is fabricated as follows. First, a catalytic reaction layer mainly composed of carbon powder carrying a platinum type metallic catalyst is formed on both sides of a polymer electrolyte membrane which selectively transports hydrogen ions. Then, a diffusion layer provided with both permeability for the fuel gas or the oxidizing gas and electron conductivity is formed on the outer surface of each catalytic reaction layer. The catalytic reaction layer and this diffusion layer integrally function as an electrode. The assembly comprising the electrodes and the electrolyte membrane called as an MEA (Membrane Electrolyte Assembly).
Then, in order to avoid the supplied gases from leaking outside the fuel cell, or the fuel gas and the oxidizing gas from mixing each other, gaskets are arranged around the electrode in such a manner as to sandwich the polymer electrolyte membrane. The gaskets may be previously formed to be integral with the electrode and the polymer electrolyte membrane, and the integrated body is called an MEA in some cases.
Furthermore, a conductive separator plate is provided outside the MEA to fix the MEA mechanically and to connect adjacent MBAs electrically in serial. At the portions of the separator plate that are in contact with the MEAs, gas channels are formed to supply reaction gases to and to carry away a produced gas or excessive gases from the surfaces of the electrodes. The gas channel could be provided individually and separately from the separator plate; however, it is common that a groove is made on the surface of the separator plate and used as the gas channel.
In general, in the actual use of a fuel cell, a laminated structure where a plurality of the above-mentioned unit cells is stacked is adopted. During the operation of the fuel cell, not only electric power but also heat is produced; in the laminated structure, a cooling plate is provided for every one or two unit cells to make it possible to keep the cell temperature constant and to use the produced heat energy in the form of hot water or the like. The common structure of a cooling plate is composed of a thin metallic plate and a heat medium such as cooling water circulating through inside thereof. Moreover, there is another structure where channels are provided on the rear side of the separator which composes a unit cell, i.e., on the side where the cooling water is intended to circulate, thereby making the separator plate function as the cooling plate. In this case, an O-ring or a gasket are needed for sealing a heat medium such as cooling water. In this sealing method, it is necessary to secure sufficient electrical conductivity between the top and bottom of the cooling plate by completely compressing the O-ring or the like.
Furthermore, in such a laminated structure, apertures called manifolds to provide and discharge the fuel gas to and from each unit cell are necessary. As the manifolds, the common one is a so-called internal manifold type provided with apertures for supplying and discharging cooling water inside the cell stack.
Which of the internal manifold type or the external manifold type may be used, it is necessary that the plurality of unit cells including cooling plates is stacked in one direction; a pair of end plates are arranged at both ends of the stacked cell; and the stacked cell is pressed and fixed from outside the two end plates by using a fastening rod. In the fastening, it is preferable to fasten the unit cells as uniformly as possible within the plane thereof. From the viewpoint of mechanical strength; the end plates and the fastening rod are usually made from a metallic material such as stainless steel. These end plates and the fastening rod are electrically isolated from the stacked cell by an insulating plate so as to produce a structure where there is no current leakage outside through the end plates. As for the fastening rod, it has been suggested to penetrate it through a through-hole inside the separator and to fasten the whole stacked cell including the end plates by means of a metallic belt.
In the aforementioned polymer electrolyte fuel cell the electrolyte membrane functions as an electrolyte when it contains some water and, therefore, it is necessary to moisturize and supply the fuel gas and the oxidizing gas. In; addition, the polymer electrolyte membrane has the effect of increasing the ion conductivity thereof with increasing water content, thereby reducing the internal resistance of the cell, and exhibiting high performance at least within a temperature range up to 100xc2x0 C. Therefore, in order to increase the water content in the electrolyte membrane, it is necessary to supply the supplied gas after being highly moisturized.
However, supplying an excessively moisturized gas at the cell operating temperature causes condensed water inside the cell, and the water drops disturb a smooth gas supply. Furthermore, on the electrode (air electrode) side to which the oxidizing gas is supplied, electric power generation produces water, thereby causing a problem that the efficiency of eliminating the produced water decreases, and the cell performance deteriorates. For this, it is general that a moisturized gas having a dew point slightly lower than the cell operating temperature is prepared and supplied into the cell.
Commonly used as methods for moisturizing a gas are a bubbler moisturizing system where a gas is supplied to deionized water maintained at a predetermined temperature and moisturized by ventilating (bubbling), and a membrane moisturizing system where deionized water maintained at a predetermined temperature is flown on one side of a membrane such as an electrolyte membrane, which allows water to move easily, whereas a gas is flown on the other side to moisturize a gas. When a gas prepared by steam reforming a fossil fuel such as methanol or methane is used as a fuel gas, there are cases where moisturizing is unnecessary because steam is contained in the reformed gas.
The moisturized fuel and oxidizing gases are supplied to the polymer electrolyte fuel cell and used for electric power generation. At this time, in a single plane of any unit cell in the stacked cell, a current density distribution occurs.
To be more specific, the fuel gas is moisturized in a predetermined manner at the gas supplying inlet and supplied; however, hydrogen in the fuel gas is consumed in electric power generation, Which causes a phenomenon that a hydrogen partial pressure is high and a steam partial pressure is low in the upstream parts of the fuel gas, whereas the hydrogen partial pressure is low and the steam partial pressure is high in the downstream parts of the fuel gas. Furthermore, the oxidizing gas is also moisturized in a predetermined manner at the gas supplying inlet and supplied; however, oxygen in the oxidant gas is consumed in electric power generation, and water is produced by electric power generation, which brings about a phenomenon that the oxygen partial pressure is high and the steam partial pressure is low in the upstream parts of the oxidizing gas, whereas the oxygen partial pressure is low and the steam partial pressure is high in the downstream parts of the oxidizing gas. Moreover, the temperature of the cooling water for cooling the cell becomes low on the inlet side and high on the outlet side of the fuel cell so as to cause a temperature distribution in a single plane of a unit cell. From these reasons, in a single plane of the cell, a current density distribution (performance distribution) occurs.
When an unevenness of the hydrogen and steam partial pressures in the fuel gas; an unevenness of the oxygen and steam partial pressures in the oxidizing gas; and a temperature distribution in a single plane of the cell, which are caused by the above-mentioned reasons become too large and go out of an optical condition, an extremely dry (over dry) condition or an extremely wet (over flooding) condition occurs, which can not be settled only by the occurrence of a current density distribution, and sometimes makes the cell fail to function as a cell.
In addition, there could be another phenomenon that an over dry and an over flooding coexist in a single plane of the cell due to an unevenness of the hydrogen and steam partial pressures in the fuel gas, an unevenness of the oxygen and steam partial pressures in the oxidizing gas, and a temperature distribution in a single plane of the cell, which are caused by the above-mentioned reasons.
When the stacking number of the stacked cell is increased, the occurrence of the above-mentioned problem in a part of the plurality of stacked unit cells will make the unit cells with decreased performance interfere with the operation of the entire stacked cell. To be more specific, when a part of the stacked unit cells falls into the over flooding condition, the cells suffering from the over flooding increase a pressure loss for gas supply.
Since the manifolds for gas supply are shared by all unit cells in the stacked cell, the presence of a unit cell fallen into the over flooding makes it harder for the gas to be supplied to the other unit cells, which may cause the over flooding to the stacked cell as a whole.
On the other hand, when a partial unit cell of the stacked cell falls into an over dry, the pressure loss for gas supply decreases in the unit cell fallen into the over dry. Consequently, the gas is flown more easily to the stacked cell fallen into an over dry, thereby causing an over dry more easily.
The above-described problem often results from the steam partial pressure in the gas becoming higher on the gas outlet side than on the gas inlet side, whether it may be on the fuel electrode side to supply the fuel gas or on the air electrode side to supply the oxidizing gas. Therefore, as disclosed in Japanese Unexamined Patent Publication No. Hei 9-511356, it has been tried to suppress the over flooding in the downstream parts of the air electrode and to reduce a current density distribution in a single plane of the cell, by making an oxidizing gas flow direction and a cooling water flow direction the same and also by making the temperature in the downstream parts of the oxidizing gas higher than in the upstream parts by means of a temperature distribution of the cooling water.
However, there is always a pressure loss at the gas inlet when a gas is supplied to the stacked cell and, therefore, there is a pressure distribution of the supplied gas inside the stacked cell, always making the inlet side higher in pressure. On the air electrode side, water is generated to make the steam partial pressure higher as the steam gets closer to the outlet side; however, the relative humidity may not necessarily be higher on the outlet side due to the influence of the pressure distribution, depending on the cell operating condition. Therefore, when the cell is powered at such an operating condition that the relative humidity closer to the inlet side gets higher; the oxidizing gas flow direction and the cooling water flow direction are made the same; and the temperature in the downstream parts of the oxidizing gas is made higher than in the upstream parts by means of a temperature distribution of the cooling water, the over flooding on the gas inlet side is facilitated, thereby producing a reverse effect.
In order to solve the above-mentioned problems, the present invention provides an method for operating a polymer electrolyte fuel cell comprising, a pair of electrodes sandwiching a polymer electrolyte membrane, a conductive separator, means for supplying and discharging a fuel gas and an oxidizing gas to and from the electrodes, a moisturizing means for the fuel gas and/or the oxidizing gas, and means for circulating cooling water in a plane direction parallel to the electrodes, the method comprising the steps of:
measuring at least one physical quantity selected from the group consisting of a gas flow rate of the fuel gas, a gas flow rate of the oxidizing gas, a saturated steam pressure in the fuel gas, a steam pressure in the fuel gas, a saturated steam pressure in the oxidizing gas, a steam pressure in the oxidizing gas, a temperature of the electrode and an output current value;
regulating at least one physical quantity selected from the group consisting of a flow direction of the cooling water, a temperature of the cooling water, a flow amount of the cooling water, a supply amount of the fuel gas, a supply amount of the oxidizing gas, a moisture amount in the fuel gas, a moisture amount in the oxidizing gas, a temperature of the electrode, a temperature distribution of the electrode and an output current value; and thereby
maintaining a property value calculated by a characteristic function using, as an independent variable, at least one physical quantity selected from the group consisting of the gas flow rate of the fuel gas, the gas flow rate of the oxidizing gas, the saturated steam pressure in the fuel gas, the steam pressure in the fuel gas, the saturated steam pressure in the oxidizing gas, the steam pressure in the oxidizing gas, the temperature of the electrode and the output current value at a predetermined value.
In this case, it is effective to make the outlet of the fuel gas and the oxidizing gas in the polymer electrolyte fuel cell to be substantially open to an ordinary pressure excluding an inevitable pressure loss of a heat exchanger or a total enthalpy heat exchanger provided at the rear stage of the outlet.
In addition, it is also effective that the characteristic function is represented by the formula (1):
Y=Vmxc3x97(xcex94P)nxe2x80x83xe2x80x83(1) 
wherein V indicates a flow rate of the fuel gas or the oxidizing gas, xcex94P is the difference between a saturated steam pressure and a steam pressure in the fuel gas or the oxidizing gas and m and n are predetermined values, and that a property value Y calculated by the formula (1) is maintained to be not less than a first predetermined value and not more than a second predetermined value.
It is also effective to regulate at least one selected from the group consisting of the first predetermined value, the second predetermined value, m and n depending on an operating duration of time or output characteristics of the polymer electrolyte fuel cell.
It is also effective to make a temperature of an electrode starting point lower than a temperature of an electrode outlet point in said electrodes, the electrode starting point being a portion into which the fuel gas or the oxidizing gas is introduced and the electrode outlet point being a portion from which the fuel gas or the oxidizing gas is discharged.
It is effective to change a temperature of the electrode from the electrode starting point to the electrode outlet point against a distance from the electrode starting point to the electrode outlet point according to a curve opening downwards.